Taken from the Warhammer 40 RPG. A much scarier version of a grell.
Contact with the creatures known only as Enslavers is thankfully rare, for they are amongst the most dangerous xenos the Imperium has ever catalogued. A trio of Enslavers can condemn a dozen or more worlds to domination and destruction, it is said, though little is known about these beings for certain. Indeed, even in situations where the Enslavers have been cited as the cause, actual contact with the creatures is scarce.
A typical Enslaver infestation is believed to follow a fairly standard progression. Initially, between one and three Enslavers will usurp the soul and body of an unprotected psyker, mutating the victim’s flesh until he is transmuted into a living gate into the Warp, through which the Enslavers can pass. These Enslavers proceed to psychically dominate any intelligent creatures they come across, employing them for all forms of manual labour and defence, while dominated psykers are commonly transformed into additional living gateways. Halting an Enslaver infestation is extremely difficult once it has begun, and only the strongest-willed individuals should ever be trusted to undertake such a task, lest they be dominated as well.
The typical Enslaver is a hideous beast, a mass of leathery flesh festooned with writhing tendrils and surrounded by the malefic aura of an entity that can dwell as easily within the Warp as within reality. They exist in defiance of the natural order of the universe, floating across battlefields and bending the minds of those who would otherwise fight them.
Aboard a Space Hulk, an Enslaver can linger in hibernation with a swarm of psychically sustained slaves for years, even decades, while the Space Hulk traverses the Warp. When their psychic senses detect a populous world, they begin preparations to infest it, to spread their psychic influence across it.
Enslavement. An enslaver may command any number of creatures it has charmed at the same time as a bonus action. Any creature who was under the Enslaver’s control and is suddenly freed from it (either through their own efforts, or because the enslaver died, or for whatever other reason) is Stunned for 1 minute as they recover their wits, and gains 1d10 Insanity Points from the experience (Or whatever equivleent used in your world to track insanity/sanity).
Magic Resistance. The enslaver has advantage on saving throws against spells and other magical effects.
Innate Spellcasting (Psionics). The enslaver’s innate spellcasting ability is Intelligence (spell save DC 15). It can innately cast the following spells, requiring no components:
Multiattack. The enslaver makes two tentacle attacks.
Tentacles. Melee Weapon Attack: +7 to hit, reach 10 ft., one creature.
Hit: 15 (2d10 + 4) psychic damage, and the target must succeed on a DC 11 Constitution saving throw or be poisoned for 1 minute. The poisoned target is paralyzed, and it can repeat the saving throw at the end of each of its turns, ending the effect on a success.
The target is also grappled (escape DC 15). If the target is Medium or smaller, it is also restrained until this grapple ends. While grappling the target, the enslaver has advantage on attack rolls against it and can’t use this attack against other targets. When the enslaver moves, any Medium or smaller target it is grappling moves with it.
Psychic Shock (Recharge 5–6). The enslaver psionically manifests psychic energy in a 60-foot cone. Each non-enslaved creature in that area must succeed on a DC 15 Intelligence saving throw or take 26 (5d8 + 4) psychic damage or half as much on a success. If all creatures targeted make their saving throw the power automatically recharges.
Psychic Song. The enslaver sings a psionic melody. Every creature within 300 feet of the enslaver that can “hear” the song must succeed on a DC 15 Wisdom saving throw or be charmed by the enslaver until the enslaver dies or until it is on a different plane of existence from the target. The charmed target is under the enslaver’s control and can’t take reactions, and the enslaver and the target can communicate telepathically with each other over any distance.
Whenever the charmed target takes damage, the target can repeat the saving throw. On a success, the effect ends. No more than once every 24 hours, the target can also repeat the saving throw when it is at least 1 mile away from the aboleth.
The enslaver may only have as many creatures charmed equal to its Wisdom ability score.